The benefits of non-invasive systems for detecting and/or monitoring for the presence of a particular analyte are well known in the art, and in particular in the biological arts. For example, in pulse oximetry a sensor is placed on a patient's body, e.g., the patient's finger. The sensor emits a red light and an infrared (IR) light sequentially through the patient, and detects the resulting light transmitted through the patient. The changing absorbance of each of the two wavelengths as the heart beats is measured and used to determine the oxygenation of the pulsing arterial blood alone. The absorbance of the red and IR light is used to calculate the oxygenation of the blood.
Methods and systems for determining and/or monitoring in vivo the oxygenation of tissue in a person or other mammal have a number of applications. For example:                During surgical procedures when a patient is under general anesthesia, it is valuable for clinicians to monitor the patient's perfusion and oxygenation. Low muscle oxygenation will provide an early warning of perfusion and oxygenation problems. An oxygenation monitor would be particularly useful during bypass surgery when pulsatile flow to the body may be limited and pulse oximetry is not a viable option.        Circulatory shock is a life-threatening medical condition that may result in inadequate oxygen to the organs and tissues of the body. Shock includes, for example, traumatic shock, cardiogenic shock, septic shock, neurogenic shock, anaphylactic shock, and the like. In a particular example, a muscle oxygenation probe would be useful in to improve triage and initial medical management, for example in battlefield situations, multi-victim accident situations, and other scenarios involving multiple casualties.        An oxygenation monitor or probe would also be beneficial for refining oxygen titration, including monitoring of neonatal (and premature infant) patients. Muscle oxygen measurement is more sensitive than other existing methods for titration of supplemental oxygen. Better modulation of oxygen support in premature infants and other patients will decrease risks of inadequate oxygen (which can result in brain or other organ damage) as well as excessive oxygen therapy (which can result in long-term lung injury).        A tissue oxygenation monitor would be beneficial in exercise physiology and sports medicine applications. In particular, muscle oxygenation is an indicator of aerobic capacity, muscle anaerobic metabolism, and performance.        Measuring muscle oxygenation during transport of patients and during pre-hospital admission as part of early evaluation could lead to earlier treatment and better outcome.        Often trauma can lead to blood vessel compression by the raised pressure within a body compartment, and ultimately to lack of oxygenation and tissue death. A tissue oxygenation probe would help identify decreasing oxygenation and increasing tissue death so treatment can be initiated early.        An in vivo oxygenation probe may be used to determine viability of tissue to be transplanted and to monitor the progress of reperfusion in the transplanted organ.        An in vivo oxygenation probe may be used to detect and/or monitor for peripheral vascular disease and/or during arterial and vascular surgery to aid in the identification of tissue perfusion status. The probe would be especially helpful in identification of tissue viability in extremities with vascular disease, for example, helping the surgeon to determine whether and where to perform amputations.        
Muscle oxygenation or saturation can be used as an early or leading signal of circulatory shock. Shock is a life-threatening medical condition characterized by “inadequate substrate for aerobic cellular respiration.” Silverman, Adam (October 2005). “Shock: A Common Pathway for Life-Threatening Pediatric Illnesses and Injuries,” Pediatric Emergency Medicine Practices 2(10). In its early stages, shock may be described generally as inadequate oxygen levels in tissue. Several conditions can trigger the onset of shock, including, for example, sepsis, heart dysfunction, and hemorrhage. If left untreated, shock can lead to Multiple Organ Dysfunction Syndrome (MODS) and even death.
A clinically important aspect of circulatory shock is that it progresses by one or more positive feedback mechanisms, and therefore can rapidly escalate to permanent damage or death if left untreated. Early detection of shock is critical to optimize patient outcomes as well as to reduce the costs for medical treatment.
Current methods of recognizing shock are indirect, non-definitive, or slow. These methods include monitoring vital signs, which are non-specific, and sampling blood chemistry, which has associated metabolic and procedural delays. High lactate levels indicate a metabolic response to the presence of shock, but they lag decreases in muscle and systemic oxygenation that occur at the onset of shock. Invasive catheters are not used to detect shock, but may be used in cases of known severe shock. These catheters are expensive and must be inserted by highly trained personnel.
Continuous measurement of muscle oxygenation would also allow physicians to refine therapy as normalization of cellular oxygenation is achieved, giving patients what they need while preventing over-resuscitation. It is well established that unnecessary blood transfusions and excess fluid administration worsen outcome.
At this time, there are several commercial near infrared spectroscopy (NIRS) devices that are being used in various areas of health and physiology. Traditional NIRS devices measure 2 to 6 discrete wavelengths in the near infrared region. Exemplary NIRS devices are disclosed in U.S. Patent Application Publication No. 2011/0205535, to Soller et al., the disclosure of which is hereby incorporated by reference in its entirety.
There remains a need for improved methods, systems, and devices for detecting and monitoring the oxygenation of tissue. There is also a clinical need for improved methods, systems, and devices for monitoring patients to rapidly detect circulatory shock.